Aluminum oxide (Al.sub.2 O.sub.3) is one of the most important raw materials in modern day industries. High purity aluminum oxide finds many important applications in the production of metallic aluminum, or for use as a substrate or insulation material in the electronic and optics industries. Conventionally, aluminum oxide can be obtained from mining bauxites. However, naturally occurring bauxites do not satisfy the quality requirement in many applications, and high grade aluminum oxide must be obtained from chemical synthesis. One of the most commonly employed techniques in making high purity aluminum oxide involves de-hydration of aluminum hydroxide [i.e., Al(OH).sub.3.3H.sub.2 O)], in a process that has been well-known as the Bayer process: EQU 2 Al(OH).sub.3 .fwdarw.Al.sub.2 O.sub.3 +3 H.sub.2 O
Aluminum hydroxide exists in various crystalline forms, or phases, including the three main phases of hydrargillite (or gibbsite), bayerite and nordstrandite. The main difference among these different phases is the arrangement of the hydroxide group (OH.sup.-) around the aluminum atom. Among the various phases of aluminum hydroxide, only the .alpha.-phase (i.e., the hydrargillite or gibbsite phase) exists naturally in large quantities. Aluminum hydroxide of the hydrargillite (or gibbsite) form is also the most stable; it constitutes the main composition of bauxite mines in the American continent. As described above, bauxite has been used traditionally in making aluminum oxide using the Bayer process. However, because of the impurities that exist in the naturally occurring bauxite, the quality of the aluminum oxide so obtained has been largely considered inadequate for high-tech use. This is true even with the naturally occurring aluminum hydroxide of the hydrargillite phase. In addition to the problem of inadequate purity, conventional processes also suffer the shortcomings of inefficient production procedure and relatively high production cost.
Aluminum hydroxide of the hydrargillite phase can also be produced synthetically. Part of the Bayer process also teaches the production of aluminum hydroxide from (1) reaction between carbon dioxide and sodium aluminate (NaAlO.sub.2) or (2) reacting sodium aluminate with water, then, in the presence of aluminum hydroxide nucleation seeds, precipitating the reaction products through nucleation and crystallization. The reaction to form aluminum hydroxide according to the Bayer process can be summarized as follows: EQU 2 NaAlO.sub.2 +CO.sub.2 +3 H.sub.2 O.fwdarw.2 Al(OH).sub.3 +Na.sub.2 CO.sub.3 ( 1) EQU NaAlO.sub.2 +2 H.sub.2 O.fwdarw.Al(OH).sub.3 +NaOH (2)
As described, the Bayer process uses sodium aluminate (NaAlO.sub.2) as a reactant to produce sodium hydroxide. The product obtained from the above process generally meets the purity standard required by the industry. However, the first process, which utilizes carbon dioxide, involves a relatively cumbersome procedure, such as preventing leaks, that is typically associated with the handling of a gaseous reactant. On the other hand, the second process requires the use of high purity aluminum hydroxide as nucleation seeds: this severely encumbers the overall efficiency of the manufacturing process.
Hydrothermal synthesis was considered the first alternative that has been proposed to prepare high purity aluminum trihydroxide. Misra and his co-workers published an article in "Journal of Crystal Growth," Vol. 8, p. 172 (1971). In that article, hydrothermal technique was employed under high temperature and high pressure conditions in an attempt to crystallize aluminum trihydroxide in gibbsite form. However, hydrothermal synthesis for years has not been seriously considered as a practical process for the mass production of any raw material, because the reaction can easily become out of control when the system pressure exceeds several hundred atmospheres.
Other processes are also disclosed in the art for the production of aluminum hydroxide. U.S. Pat. No. 4,612,184 discloses a process for the preparation of high specific surface hydrargillite by first reacting an alkali metal aluminate with hydrofluoric acid, then filtering and washing the resultant aluminum hydroxide precipitates. U.S. Pat. No. 5,225,229 discloses a method for the production of aluminum hydroxide by reacting water in the liquid phase with aluminum powder at a pH above about 12.4, in the presence of chlorine. An alpha alumina promotor is added to the reaction mixture as the aluminum hydroxide is precipitating. These processes provided some improvements over the classical Bayer process, but they also introduced other drawbacks. Most notably, hydrofluoric acid is well-know to be very corrosive against glass containers, and its transportation, especially on a large scale and having to cross interstate lines, can create significant logistic nightmares. Therefore, although the use of hydrofluoric acid in the place of carbonic acid eliminates the need of using aluminum hydroxide nucleation seeds, it is not a practical substitute, as far as industrial scale production of aluminum hydroxide is concerned.